1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.
The channels in a wireless communication system are defined by one or more transmission protocols. For example, in wireless communication systems that operate according to the Frequency Division Multiple Access (FDMA) protocol, the transmission bandwidth allocated to the air interface is divided into a number of frequencies and these frequencies are allocated to the various channels. For another example, Code Division Multiple Access (CDMA) protocols implement coding sequences that may be used to modulate transmitted information so that multiple users may transmit on the same frequency or set of frequencies. Other transmission protocols include Orthogonal Frequency Division Multiple Access (OFDMA) and Single Channel-FDMA (SC-FDMA). In an OFDMA system, the available bandwidth may be divided into a plurality of orthogonal subcarrier frequencies (commonly referred to as subcarriers), which may be allocated to one or more subchannels so that multiple users may transmit data concurrently using separate groups of subchannels. In SC-FDMA, the available bandwidth is also divided into a plurality of orthogonal subcarriers similar to OFDMA, but discrete Fourier transform (DFT) pre-coding is used to provide low Peak-to-Average-Power Ratio (PAPR) compared with OFDMA transmission.
Wireless communication systems that operate according to OFDMA protocols also implement scheduling of data transmissions on the uplink. Scheduling refers to allocating time-frequency resource(s) of the wireless communication system for data transmission. Scheduling involves transmitting a scheduling request in the uplink and a scheduling grant in the downlink before data transmission. Scheduling increases latency and overhead in the wireless communication system. Scheduling data transmission is not typically desirable, or even always possible, for traffic that is delay sensitive and/or has small packet sizes, such as Voice over Internet Protocol (VoIP), Layer-1 control signaling, Layer-2 control signaling, and the like. Consequently, in Evolved UMTS Terrestrial Radio Access (E-UTRA) system, SC-FDMA with discrete Fourier transform (DFT) precoding was introduced in the uplink. Orthogonality of in-cell users is achieved by using OFDM in the presence of frequency-selective fading. Furthermore, DFT precoding was introduced to reduce the large Peak-to-Average-Power-Ratio (PAPR) of the pure OFDM transmission. With the addition of higher-order modulation and adaptive modulation coding (AMC), large spectral efficiency is possible by scheduling users with favorable channel conditions. However, to realize the large spectral efficiency of SC-FDMA system, fast scheduling in time and frequency is necessary. This may add a large amount of scheduling overhead for both the uplink and the downlink control channels, and may be inefficient when supporting a large number of delay-sensitive and/or low data rate traffic flows. One possible approach for supporting low data rate traffic flows is to semi-statically allocate time-frequency zones or to implement interference avoidance schemes. However, these approaches may suffer from low channel utilization.